The invention relates to dead weight piston fluid pressure gauges/calibration standards, and more particularly to high pressure dead weight piston gas pressure gauges/calibration standards.
Dead weight piston pressure measurement/calibration devices are well-known. Such devices ordinarily include a piston supporting a selected number of calibration weights. A xe2x80x9cdead weight piston assemblyxe2x80x9d includes the piston, a bell housing, and the calibration weights. The piston is slidably disposed in a cylinder, in very low frictional relationship to the cylinder. Fluid, which can be gas or liquid the pressure of which is to be defined or measured, is metered into the bottom of the cylinder so as to push the dead weight piston assembly upward. When the force produced on the bottom of the piston by the pressurized fluid equals the weight of the dead weight piston assembly, the dead weight piston assembly xe2x80x9cfloatsxe2x80x9d in an equilibrium condition, wherein a downward force exerted by the piston and supported by the pressure is equal to the total mass multiplied by the gravitational constant. The opposing upward force is produced by the pressure Pg of the gas being measured against the effective area of the piston-cylinder assembly.
Generally, the piston supports a hollow, cylindrical bell housing that in turn supports the annular weights to be loaded on the piston. A horizontal annular flange or ledge is attached to and extends outwardly from a lower outer surface of the bell housing, and one or more annular weights typically are loaded on the annular flange. The piston, bell housing, and annular weight assembly is very precisely and symmetrically shaped and balanced. A selected number of the calibration weights are stacked on the ledge for the purpose of precisely establishing the total weight of the dead weight piston assembly. A spin then is imparted to the dead weight piston assembly, which is sufficiently symmetrical about the vertical axis of the piston that the piston spins freely within the cylinder, the outer surface of the piston being lubricated from the walls of the cylinder by a thin layer of fluid, which can be gas or liquid. The known weight of the dead weight piston assembly and the known xe2x80x9ceffective areaxe2x80x9d of the xe2x80x9cpiston-cylinderxe2x80x9d are used to precisely compute the pressure of the fluid being supplied to support the dead weight piston assembly in a xe2x80x9cfree-floatingxe2x80x9d equilibrium condition between upper and lower stops of the dead weight piston calibration device.
The closest prior art is thought to include (1) commonly assigned U.S. Pat. No. 5,331,838 entitled xe2x80x9cDEAD WEIGHT PISTON DRIVE AND CONTROL SYSTEMxe2x80x9d, by Delajoud, issued Jul. 26, 1994, (2) the device shown in FIG. 1, described below, (3) and the xe2x80x9cintegrated piston-cylinder metrological modulesxe2x80x9d used in the assignee""s PG 7000 line of piston gauge products. Above-mentioned U.S. Pat. No. 5,331,838 is incorporated herein by reference.
Gas is much less viscous than oil. Consequently, unless the gap between the rotating piston and the cylinder is very small (0.3 to 0.8 microns), using gas as the lubricant in the gap results in the rotating piston not being as well centered within the cylinder as if oil is used as the lubricating fluid. It is extremely difficult to maintain such a small gap at high pressure due to manufacturing constraints and deformation of the piston and cylinder with respect to pressure. Therefore, pressurized oil normally is used to lubricate the gap between the rotating piston and cylinder for high pressure operation. However, there is a need for piston gauges operating at high pressure using gas as the test medium. Due to the difficulty of lubricating the piston-cylinder with gas at high pressure, the conventional approach is to use an oil operated piston gauge combined with an oil to gas interface external from the piston gauge. However, this method adds uncertainty to the value of the gas pressure due to lack of knowledge of the exact level of the oil to gas interface and is impractical to operate due to the need to maintain the oil to gas level when changing the pressure.
Approximately 20 years ago a French company named Desgranges et Huot developed the system shown in xe2x80x9cprior artxe2x80x9d FIG. 1 to solve the problems associated with the use of high pressure gas in a piston gauge by xe2x80x9cindirectlyxe2x80x9d lubricating the gap between the piston and cylinder with oil. The main benefits of the system of xe2x80x9cprior artxe2x80x9d FIG. 1 are (1) that the piston-cylinder gap can be a larger size that works well with oil piston gauges, (2) that the xe2x80x9cdrop ratexe2x80x9d of the piston is much lower than with gas, since the viscosity of oil is higher than the viscosity of the gas being measured, and (3) that the operation of the piston-cylinder is unaffected by the cleanliness of gas under the piston.
Referring to FIG. 1, the pressurized gas to be measured is introduced through passage 43 into volume 42, and exerts upward force on the bottom of rotating piston 23, which supports a mass (not shown) supported by piston head 10. The pressure Pg of the gas to be measured is transmitted through a tube 44 to the top of a small oil reservoir 45 containing lubricating oil 46.
The bottom of oil reservoir 45 is coupled by a tube 47 through the wall of cylinder 16 between two O rings 48 and 49 into the approximately 1 micron gap between the vertical wall of piston 23 and the wall of cylinder 16. The top level of the oil 46 in reservoir 45 is located a distance h above the point at which the channel 47 enters the gap, so the pressure of a column of the oil 46 always is added to the gas pressure Pg and ensures that none of the pressurized gas enters into the gap. The distance h is large enough that the head pressure of the oil 46 ensures that the oil pressure is higher than the gas pressure under the piston so that there is a slight flow of oil out the bottom end of the gap as indicated by arrow 50B, thus preventing any of the high-pressure gas from displacing oil in the gap. The main flow of oil out of the upper end of the gap as indicated by arrow 50A is produced by the addition of the gas pressure Pg and the head pressure of the oil.
The system of FIG. 1 provides gas pressure measurements at the relatively low levels of accuracy that were needed 10 to 20 years ago. However, a problem of the system of FIG. 1 is that in order to change the range of pressures of gas to be measured, it often is necessary to interchange the piston assembly 23, and the cylinder 16. To accomplish this interchanging for the device of FIG. 1, it is necessary to first remove the piston assembly 23,10, and then remove the cylinder 16. However, when piston 23 is removed, the head pressure of oil 46 in reservoir 45 causes a relatively large amount of the oil to leak out by flowing through channel 47 into the volume left open by the removal of piston 23, and the large flow of oil continues after cylinder 16 is removed. That is quite problematic, because the oil in volume 42, if not purged, may contaminate the gas which is measured next after another cylinder and piston have been installed. But purging the oil from volume 42 is time-consuming and costly, and also can pollute a laboratory environment with oil vapor. When oil 46 leaks or must be drained from reservoir 45, it may be excessively time-consuming and expensive to refill the reservoir with the special oil which may be required.
A much larger problem associated with the piston gauge shown in FIG. 1 is that for very high gas pressures, e.g., for Pg greater than approximately 7 MPa (1000 psi), the structure causes deformation of both piston 23 and cylinder 16. The deformation of the cylinder that occurs is difficult or impossible to model mathematically, so the pressure deformation coefficient of the piston-cylinder assembly cannot be accurately mathematically computed. This makes it difficult or impossible to accurately determine the variation of xe2x80x9ceffective areaxe2x80x9d of the piston-cylinder 23,16 with respect to pressure. The only way to determine the variation of the xe2x80x9ceffective areaxe2x80x9d with pressure is by comparison to an oil pressure standard for which the deformation coefficient is well known.
The mounting system for cylinder 16 within the housing 31 in FIG. 1 results in the very high pressure Pg (of the gas being measured) being applied directly on the portions of the surfaces of piston 23 and cylinder 16 below O ring 48. However, only the low ambient pressure Pa is exerted at the top of cylinder 16 and on the portion of the outside surface of cylinder 16 located above O ring 48. Thus, there is an extremely abrupt drop in the pressure exerted across the wall of cylinder 16 (e.g., up to 70 to 100 MPa (10,000 to 15,000 psi)) from a location just below to a location just above O ring 48. That abrupt pressure drop causes cylinder 16 to deform unpredictably, i.e., in a way that is not possible to accurately model mathematically. That makes it very difficult or impossible to accurately compute the effective area of piston-cylinder 23,16. Another factor that further increases the uncertainty in the knowledge of the pressure deformation coefficient, and hence the xe2x80x9ceffective areaxe2x80x9d of piston 23 (also referred to as the xe2x80x9ceffective areaxe2x80x9d of piston-cylinder 23,16), is non-reproduceability of the deformation coefficient that occurs due to slight displacement in the positions of the O rings when the cylinder is removed and replaced.
Since the measurement of the pressure Pg is determined by multiplying the total mass of piston 23 and the other mass supported thereon by the gravitational constant g, divided by the xe2x80x9ceffective areaxe2x80x9d of piston-cylinder 23,16, the system of prior art FIG. 1 is incapable of providing the accurate measurements of gas pressures above roughly 7 MPa (1000 psi) needed for many current applications.
Thus, there is an unmet need for an improved piston gauge which is capable of accurate measurement of very high gas pressures, e.g. above roughly 7 MPa (1000 psi). There also is a need for an improved very high pressure piston gauge which avoids the above described abrupt deformation of the cylinder.
Accordingly, it is an object of the invention to provide an interchangeable piston-cylinder module for a piston-cylinder-based pressure measurement gauge that is capable of accurately measuring very high gas pressures.
It is another object of the invention to provide an interchangeable piston-cylinder module for a dead weight piston gauge or the like that is capable of defining very high gas pressures, up to approximately 100 MPa (15,000 psi), with very low measurement uncertainty, e.g., less than +xe2x88x9230 ppm of the measured pressure.
It is another object of the invention to provide an interchangeable piston-cylinder module for a dead weight piston gauge or the like that is capable of accurately measuring very high fluid pressures.
It is another object of the invention to provide an interchangeable piston-cylinder module for a dead weight piston gauge or the like that is capable of accurately measuring gas pressure at very high pressures, for example as high as 100 MPa (15,000 psi) or more.
It is another object of the invention to provide a piston-cylinder module for a dead weight piston gauge or the like which avoids inaccuracy in measurement of high gas or liquid pressure due to very non-ideal deformation of the cylinder caused by very high gas or liquid pressure to be measured.
It is another object of the invention to provide a piston-cylinder module for a dead weight piston gauge or the like which avoids contamination that occurs in the system itself and the device or devices it may be connected to for prior art dead weight piston gauges due to flow of lubricating oil or other fluid when the piston-cylinder is removed, for example to interchange it with a more suitable piston-cylinder.
It is another object of the invention to provide a piston-cylinder module for a dead weight piston gauge or the like which conveniently allows different fluids to be used to lubricate the gap between the piston and the cylinder so that oil of different viscosities can be used, depending on gap size and for special applications (e.g., for fluorinated oil for oxygen service).
It is another object of the invention to provide a piston-cylinder module for a dead weight piston gauge or the like which can operate with either gas or liquid as the pressurized medium.
It is another object of the invention to provide a piston-cylinder module mounting post that can be operated with either liquid lubricated, gas operated piston-cylinder modules or liquid operated piston-cylinder modules.
Briefly described, and in accordance with one embodiment thereof, the invention provides a technique for accurately measuring the pressure (Pg) of very highly pressurized fluid, by providing a cylinder (16) having a cylindrical outer surface (16A) and a bore extending through the cylinder and an elongated piston (23) rotatable and vertically movable in the bore, and supporting calibration weights by means of the piston. An internal reservoir (32) is formed by providing a housing (31) to support the cylinder. The housing includes a lower portion engaging a bottom portion of the cylinder and a lower peripheral portion of the cylinder, and also includes an upper portion engaging an upper peripheral portion of the cylinder, an inner portion of the housing (31) and a portion of the outer surface (16A). The interior of the reservoir (32) is pressurized to the high pressure (Pg) of the fluid through a first passage (30A) extending from an upper portion of the reservoir to an inlet opening (31B) of the housing (31) coupled to receive the highly pressurized fluid. A first O ring (33) forms a first seal between a bottom surface of the cylinder and the lower portion of the annular housing (31), and a second O ring (39) forms a second seal between a top surface of the cylinder and a flange (29) associated with an upper portion of the annular housing (31). A quantity of oil (46) of suitable viscosity is provided in the reservoir. The first passage (30A) extends from an upper portion of the reservoir above the surface of the oil to the inlet opening (31B). A second passage (34) extends from a lower portion of the reservoir below the surface of the oil through a wall of the cylinder into the bore to conduct pressurized oil into a gap between the piston (23) and the bore.
The invention is described in an embodiment including an interchangeable module (100) for use in a dead weight piston pressure measurement device, wherein the interchangeable module includes a cylinder (16) having a cylindrical outer surface (16A) and a bore extending through the cylinder, an elongated piston (23) rotatable and vertically movable in the bore, a piston supporting calibration weights, and an annular housing (31) for supporting the cylinder, the housing including a lower portion engaging a bottom portion of the cylinder and a lower peripheral portion of the cylinder and an upper portion engaging an upper peripheral portion of the cylinder, an inner portion of the housing (31) and a portion of the outer surface (16A) forming an annular internal oil reservoir (32). A quantity of oil (46) is held in the oil reservoir. The first passage (30A) extends from an upper portion of the oil reservoir above the surface of the oil to an inlet opening (31B) to pressurize the reservoir at the pressure (Pg) of a fluid to be measured, and a second passage (34) extends from a lower portion of the oil reservoir below the surface of the oil through a wall of the cylinder into the bore to conduct pressurized oil into the gap between the piston (23) and the bore. A first O ring (33) forms a first seal between a bottom surface of the cylinder and the lower portion of the annular housing (31), and a second O ring (39) forms a second seal between a top surface of the cylinder and a flange (29) associated with an upper portion of the annular housing (31). In the described embodiment, the third O ring (35) forms a seal between the flange (29) and the upper portion of the annular housing. In the described embodiment, the first O ring 33 is disposed concentrically along a peripheral portion of the bottom surface of the cylinder (16), and the second O ring 39 is disposed concentrically along a peripheral portion of the top surface of the cylinder, and the first O ring (33) is disposed on the cylinder symmetrically relative to the second O ring (39). The pressure of the fluid to be measured is greater than approximately 100 kPa (15 psi), and wherein ambient atmospheric pressure (Pa) is present at an edge of the gap at the upper surface of the cylinder, wherein a pressure equal to the pressure (Pg) of the fluid to be measured is exerted uniformly against the portion of the cylindrical outer surface (16A) forming the oil chamber (32), and wherein pressure on the surface of the bore of the cylinder (16) varies gradually from a second passage (34) to the upper surface of the cylinder so that no abrupt deformation of the cylinder occurs due to the pressure of the fluid to be measured.